1. Field of the Invention
The present invention relates to a silent chain including a number of link plates articulately connected together by pins and each having a pair of pin-accommodating holes receiving therein the pins and a pair of teeth for meshing engagement with teeth of a sprocket.
2. Description of the Related Art
In general, a silent chain includes joint trains of link plates and guide trains of link plates arranged alternately in the longitudinal direction of the chain. The joint trains and guide trains each have a plurality of link plates connected together in an interleaved fashion by means of connecting pins. The connecting pins may be a round pin or a rocker pin composed of a pair of pins of different lengths. The link plates each have a pair of teeth for meshing engagement with sprocket teeth and a pair of pin-accommodating holes in which the connecting pins are inserted for securing articulating movement of the adjacent link plates. Each of the guide link trains has two guide plates disposed on opposite outer sides thereof. In a special type of silent chain, link plates of the guide trains do not have teeth meshing with the sprocket teeth.
The link plates are produced by punching pin-accommodating holes and the contour of link plate out from a blank sheet either simultaneously or in succession using a power press. The punched or press-formed link plates have a pair of teeth on one side thereof each formed by an inside flank and an outside flank converging together at the tip of each tooth. The inside flank or the outside flank serves as an engagement surface for meshing engagement with teeth of a sprocket. The engagement surface has a rounded shear drop or depression, a smooth sheared surface, and a rough broken or ruptured surface formed contiguously across the thickness of the plate in the order named as viewed from the direction of punching.
In the link plates formed by the conventional punching process, the proportion of smooth sheared surface is about 20% the thickness of the plate. If such link plates are assembled into a silent chain, meshing engagement between the teeth of the link plates and teeth will produce a great contact pressure resulting in the occurrence of pitching wear on the sprocket teeth To deal with this problem, the engagement surfaces of the press-formed link plates are subjected to a post-treatment, such as shaving or fine-blanking, so as to increase the proportion of sheared surface up to 70% or greater of the thickness of the link plates. By thus increasing the proportion of sheared surface, the contact area between the link plate teeth and the sprocket teeth is enlarged to thereby reduce sear of the sprocket teeth (see Japanese Patent Laid-open Publication No. HEI-9-250599).
The link plate teeth having respective shear surfaces across more than 70% of the thickness of the link plate, however, give rise to a problem in that while the silent chain is traveling between two sprockets, impact or collision sound is generated at the onset of meshing engagement between the link plate teeth and the sprocket teeth, increasing the noise level during operation of a silent chain drive. This problem is not negligible, because the silent chain is essentially used in an application in which reduction of the operation noise is a major requirement.
A study and investigation made by the present inventors established the causality between the proportion of sheared surface and the noise level. More specifically, the noise level becomes high as the proportion of sheared surface exceeds 50% of the thickness of the link plate with respect to all of the link plates. This is due to a considerable increase in the rigidity of a meshing engagement portion of the link plate which is adapted to be meshed with the sprocket teeth.